Everything Science Forum
Everything Space => Space Science and Astronomy => Topic started by: Astronuc on August 25, 2005, 05:32:22 AM
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Supernovae are interesting and spectacular phenomena.
Here are some references:
Wikipedia - Supernova (http://en.wikipedia.org/wiki/Supernova)
Supernova (http://www.astro.rug.nl/~onderwys/ACTUEELONDERZOEK/JAAR2001/jakob/aozindex.html) - What are the progenitors of the supernova type 1A explosion?
Jakob van Bethlehem
See also Supernovae (http://www.everything-science.com/components/com_smf/index.php?topic=5849.msg54580#msg54580) thread in "Ask the Astronomer".
Cosmic Rays and Supernova Source (http://www.everything-science.com/component/option,com_smf/Itemid,82/topic,5608.0) in this forum.
Related thread 18 Supernovas Discovered by The Subaru Telescope (http://www.everything-science.com/component/option,com_smf/Itemid,82/topic,735.0)
SNEWS: SuperNova Early Warning System
http://snews.bnl.gov/
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(AOL/AP/NASA) - This titanic explosion took place in the Large Magellanic Cloud, a nearby neighbor galaxy some 160,000 light-years away.
STScI-PRC2005-30 - NASA Space Observatories Glimpse Faint Afterglow of Nearby Stellar Explosion
Intricate wisps of glowing gas float amid a myriad of stars in this image created by combining data from NASA's Hubble Space Telescope and Chandra X-ray Observatory. The gas is a supernova remnant, cataloged as N132D, ejected from the explosion of a massive star that occurred some 3,000 years ago. This titanic explosion took place in the Large Magellanic Cloud, a nearby neighbor galaxy of our own Milky Way.
The complex structure of N132D is due to the expanding supersonic shock wave from the explosion impacting the interstellar gas of the LMC. Deep within the remnant, the Hubble visible light image reveals a crescent-shaped cloud of pink emission from hydrogen gas, and soft purple wisps that correspond to regions of glowing oxygen emission. A dense background of colorful stars in the LMC is also shown in the Hubble image.
The large horseshoe-shaped gas cloud on the left-hand side of the remnant is glowing in X-rays, as imaged by Chandra. In order to emit X-rays, the gas must have been heated to a temperature of about 18 million °F (10 million °C). A supernova-generated shock wave traveling at a velocity of more than four million miles per hour (2,000 kilometers per second) is continuing to propagate through the low-density medium today. The shock front where the material from the supernova collides with ambient interstellar material in the LMC is responsible for these high temperatures.
It is estimated that the star that exploded as a supernova to produce the N132D remnant was 10 to 15 times more massive than our own Sun. As fast-moving ejecta from the explosion slam into the cool, dense interstellar clouds in the LMC, complex shock fronts are created.
A supernova remnant like N132D provides a rare opportunity for direct observation of stellar material, because it is made of gas that was recently hidden deep inside a star. Thus it provides information on stellar evolution and the creation of chemical elements such as oxygen through nuclear reactions in their cores. Such observations also help reveal how the interstellar medium (the gas that occupies the vast spaces between the stars) is enriched with chemical elements because of supernova explosions. Later on, these elements are incorporated into new generations of stars and their accompanying planets.
Visible only from Earth's southern hemisphere, the LMC is an irregular galaxy lying about 160,000 light-years from the Milky Way. The supernova remnant appears to be about 3,000 years old, but since its light took 160,000 years to reach us, the explosion actually occurred some 163,000 years ago.
This composite image of N132D was created by the Hubble Heritage team from visible-light data taken in January 2004 with Hubble's Advanced Camera for Surveys, and X-ray images obtained in July 2000 by Chandra's Advanced CCD Imaging Spectrometer. This marks the first Hubble Heritage image that combines pictures taken by two separate space observatories. The Hubble data include color filters that sample starlight in the blue, green, and red portions of the spectrum, as well as the pink emission from glowing hydrogen gas. The Chandra data are assigned blue in the color composite, in accordance with the much higher energy of the X-rays, emitted from extremely hot gas. This gas does not emit a significant amount of optical light, and was only detected by Chandra.
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A rather spectacular site - Vela supernova remnant - images from Anglo-Australian Observatory
A supernova event marks the catastrophic end-point in the life of a massive star. The self-destruction of the star releases a huge amount of energy as radiation of all kinds, but a substantial fraction of the force of the explosion blasts the outer part of the supernova into an expanding shell of matter that travels through the almost empty space between the stars. As the rapidly moving blast wave encounters the relatively stationary interstellar medium it creates a very narrow, luminous shock front that appears as a faint nebula. Because the shock front is very convoluted the nebulosity appears to be highly structured and the various colours define different energy levels within the interaction. Though the star responsible for the Vela supernova remnant exploded 12,000 years ago, it is still affecting its environment.
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http://www.rochesterastronomy.org/SNIMAGES/
http://heasarc.gsfc.nasa.gov/docs/snr.html
Hidden Star Explains Supernova Oddity
by Tariq Malik
http://www.space.com/scienceastronomy/060504_supernova_companion.html
(Space.com has adds and popups >:( :P )
A star once hidden by a stellar death shroud is the source of odd behavior of its companion supernova, a new study has found.
The find has laid to rest lingering questions over how the supernova, known as SN2001ig, seemed to change its cosmic stripes within weeks while astronomers looked on.
"It was quite satisfying to be able to hold up a picture and see that there was a companion star right where we predicted," said Stuart Ryder, an astronomer with the Anglo-Australian Observatory in Sydney, Australia, in a telephone interview.
Ryder and his team suspected a companion star was to blame for SN2001ig's shift from a hydrogen-rich supernova, known as a Type II, to a Type I variety devoid of detectable hydrogen, based on previous radio observations. But it was only after debris from the initial stellar explosion cleared that the star's existence, and its likely role in stripping hydrogen from the supernova, were confirmed by optical observations.
Bright Supernovae (2001) -
http://ftp.ru.postgresql.org/mirrors/isn/sn2001/index.html
http://ftp.ru.postgresql.org/mirrors/isn/sn2001/upd01.html
Edit (05-11-06): Companion Explains "Chameleon" Supernova
http://www.gemini.edu/index.php?option=content&task=view&id=184
Australian astronomers have found a predicted "companion" star left behind when its partner exploded as a very unusual supernova. The presence of the companion explains why the supernova, which started off looking like one kind of exploding star, seemed to change its identity after a few weeks.
Renowned Australian supernova hunter Bob Evans first spotted supernova 2001ig in December 2001. It lies in the outskirts of a spiral galaxy NGC 7424, which is about 37 million light-years away in the southern constellation of Grus (the Crane).
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I'd like to see a little more explanation about how the spectral type changes, I know space.com isn't the best place to find that kind of detail. ;) From what I can discern, I assume that it's the usual model, meaning that the companion star was dumping hydrogen on the supernova pregenitor which caused the mass of the star to rise and the carbon core to begin thermonuclear fusion. So if I'm reading this right, hydrogen being blown away from the top layers would have been observed at the very beginning of the event but not later.
I guess it's also implied that this would be a common pattern for Type I (I think actually Type Ia) supernovae but by the nature of such events they are generally not seen at the very beginning of their detonations.
I've seen it theorized that this class of supernovae usually blow their companion stars away when they ignite, which is why its been hard to find direct evidence for the companion star model.
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Some good comments and observations.
Here is an article from ESA on supernovae, particularly one SN 1993J, where there is sufficient observational evidence to elucidate the behavior of a companion.
http://hubble.esa.int/science-e/www/object/index.cfm?fobjectid=34455
The second brightest supernova discovered in modern times, SN 1993J, was found in the beautiful spiral galaxy M81 on 28 March 1993. From archival images of this galaxy taken before the explosion, a red supergiant was identified as the mother star in 1993 - only the second time astronomers have actually seen the progenitor of a supernova explosion (the first was SN 1987A, the supernova that exploded in 1987 in our neighbouring galaxy, the Large Magellanic Cloud). Initially rather ordinary, SN 1993J began to puzzle astronomers as its ejecta seemed too rich in the chemical element helium and instead of fading normally it showed a bizarre sharp increase in brightness. The astronomers realised that a normal red supergiant alone could not have given rise to such a weird supernova. It was suggested that the red supergiant orbited a companion star that had shredded its outer layers just before the explosion.
I need to dig further into the spectral type of the progenitor (a red supergiant) and then look into other similar binaries, and then see what has been observed in the last two years.
Meanwhile - "A method to identify the companion stars of type Ia supernovae in young supernova remnants," Jin Ozaki, Toshikazu Shigeyama
http://arxiv.org/PS_cache/astro-ph/pdf/0602/0602574.pdf
Some more background on supernovae - http://www.pha.jhu.edu/~qzhang/seminar1bfigs/seminar1bfigs.html
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I looked at a book (popular science category) on cosmic catastrophes this weekend. I'll post the reference in a bit. There are many sites that discuss the various supernova phenomena. Here's a Wikepedia link (more technical detail is available elsewhere.)
http://en.wikipedia.org/wiki/Supernova
Drawing from my memory of what I read here's a simple breakdown of the supernovae types.
Type Ia-detonation of a carbon core white dwarf star-mechanism not confirmed but presumed to occur from some kind of mass addition-perhaps from a companion star or from a collision with another white dwarf. Spectral characteristics-no hydrogen, no helium, strong silicon line at maximum, large amount of iron at later stages.
Type Ib-detonation of a massive star due to core collapse. Spectral characteristics-no hydrogen, but helium, absence of strong silicon line, iron present. Presumably this is due to a large star collapsing which had its outer envelope of hydrogen stripped before the collapse.
Type Ic-detonation of a massive star due to core collapse. Spectral characteristics-no hydrogen, no helium, absence of strong silicon line, iron present. Like Type Ib, the hydrogen envelope is presumed to have been stripped prior to the collapse but here the helium shell is gone too.
Type II-detonation of a massive star due to core collapse. Spectral characteristics-hydrogen, helium present, as are a whole range of light to very heavy elements. The progenitor star is presumed to have retained all the shells of its history of fusing heavier and heavier elements until it comes to a stop at iron.
The Type Ia's are the really distinctive ones-what they call a deflagration ignition in a relatively small stellar remnant.
Articles like the space.com one really frustrate me. I'd like to know more about what the supernova type is theorized to have been.
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I agree about space.com articles - they don't have much in the way of substantive discussion, partricularly the physics. Spacedaily articles are better, but I usually have to contact the person doing the research, or track down relevant articles which have been published.
I am also very curious about supernovaes and the subsequent remnants - nebulae and neutron stars - and the companion stars.
I'd like to know more about the binary star systems, and the type of stars which go supernovae.
It would be interesting to be able to predict which stars might go SN.
There is also a thread on Magnetars and Neutron Stars (http://www.everything-science.com/component/option,com_smf/Itemid,82/topic,6865.0). I have to wonder if the initial nova or supernova was observed.
Unfortunately, there aren't any good records of stellar spectra going back hundreds or thousands of years, and I am not sure if we have sufficient data on magnitudes of stars that went SN.
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Among presumed core collapse supernovae, there is the well known example of the Crab Nebula which contains a pulsar-that star detonated almost a thousand years ago.
Then there's the famous example of Supernova 1987A which was observed in the Large Magellenic Cloud nineteen years ago. It appears to have been a core collapse type. It's a great story how the burst of neutrinos was detected by the Kamiokande detectors before anyone knew the star had exploded.
http://hep.bu.edu/~superk/gc.html
According to the core collapse model, a neutron star should have been left after the explosion. The mystery that continues to this day is why a neutron star-pulsar still hasn't been observed at the center of the explosion.
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Retrospector, if you have have links to other sites or papers, particularly more technical detail, please post them.
Meanwhile, I found a list of supernovae -
http://cfa-www.harvard.edu/cfa/ps/lists/Supernovae.html
List of Supernovae - gives details on all supernovae reported since 1885, as well as four earlier galactic supernovae. All coordinates given in the table below are J2000.0 positions. Date refers to the date of discovery, Mag. to the magnitude at discovery and Offset to the offset from the nucleus of the host galaxy as reported at time of discovery. Disc. Ref. is the reference to the discovery report, Posn. Ref. is the source of the accurate position and Type is the supernova's type (as reported at or near the time of discovery). Note that the preferred hierarchy for host-galaxy catalogue designations is M/NGC/IC, UGC, MCG; all other catalogues are considered "Anon" below.
Well this answers some questions I had as to where, when and what kind of SN. ;D
and this -
While many supernovae have been seen in nearby galaxies, they are relatively rare events in our own galaxy. The last to be seen was Kepler's star in 1604. This remnant has been studied by many X-ray astronomy satellites, including ROSAT. There are, however, many remnants of Supernovae explosions in our galaxy, that are seen as X-ray shell like structures caused by the shock wave propagating out into the interstellar medium. Another famous remnant is the Crab Nebula which exploded in 1054. In this case a pulsar is seen which rotates 30 times a second and emits a rotating beam of X-rays (like a lighthouse). Another dramatic supernova remnant is the Cygnus Loop.
from http://heasarc.gsfc.nasa.gov/docs/snr.html
Now I have some questions to formulate and some information to sift through.
I would like to understand more about the precursor stars. I wonder what fractions of supernovae involve precursors in a binary system.
And more resources on the web regarding supernovae -
http://rsd-www.nrl.navy.mil/7212/montes/sne.html
:koala
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Brief overview of supernovae on hyperphysics - http://hyperphysics.phy-astr.gsu.edu/HBASE/astro/snovcn.html
Formation of heavy elements
http://hyperphysics.phy-astr.gsu.edu/HBASE/astro/nucsyn.html#c1
Supernova 1987A
http://hyperphysics.phy-astr.gsu.edu/HBASE/astro/sn87a.html#c1
The discovery of Supernova 1987A was a great opportunity for detailed observation of the dynamics of a supernova. This supernova was unusual in that it did not rise abruptly to its maximum brightness, but paused briefly before increasing its luminosity by another factor of ten. It increased for about 85 days and then began a gradual decline which was more characteristic of other observed supernovae.
Another unique feature about this supernova is that the star which exploded had been observed and named Sk-69202. It was a blue supergiant presumed to have a mass of about 20 solar masses.
and this reference of Harvard's Chandra site -
http://chandra.harvard.edu/xray_sources/supernovas.html
Interestingly this page - http://chandra.harvard.edu/resources/illustrations/superNeutron.html -shows a red giant, which explodes and forms a neutron star.
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Here's a list of supernovae in our Milky Way Galaxy
http://www.seds.org/messier/more/mw_sn.html
Information on the historical supernovae - Michael Richmond (March 19, 1997)
http://stupendous.rit.edu/richmond/answers/historical.html
Types - http://a188-l009.rit.edu/richmond/answers/sntypes.txt
HISTORICAL RECORDS OF SUPERNOVAE
http://astrosun2.astro.cornell.edu/academics/courses//astro201/sn_history.htm
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Astronuc, I've seen some good links and a lot of pictures in recent years. I need to trace them and organize some posts.
It's a bit of a mystery why no naked eye supernova has been observed in our home galaxy for more than four centuries. The only naked eye supernova that has occurred since that time occurred in 1987, in the Large Magellenic Cloud (close to the Milky Way, but no cigar.) Some astronomers speculate that since there is so much dust between us and the inner regions of the galaxy, that is what has blocked the naked eye views of many supernova events in the Milky Way (although there are other ways now to detect the explosions themselves.)
There is relatively nearby supernova remnant, it is a radio source known as Cassiopeia A. It is in our galaxy about 11,000 ly away, relatively close, and the mystery is why it wasn't observed by anyone as a naked eye supernova in the late 17th Century. Here's a short link:
http://www.seds.org/~spider/spider/Vars/casA.html
The remnant really looks like an explosion! And there is what may be a neutron star at the centre.
(http://www.seds.org/~spider/spider/Vars/Pics/casA_cxo.jpg)
This is thought to have been a supernova resulting from a core collapse, such as the one which presumably created the Crab Nebula.
The supernovae of most interest in recent years have been the Type Ia's, which are thought to result from the explosion of white dwarf stars in binary systems. They appear to serve as "standard candles", objects whose absolute brightness can be judged from their luminosity-time characteristics and therefore allow distances to their parent galaxies-even over billions of light years-to be accurately judged.
What historical supernova events in our galaxy were Type Ia is an extremely interesting topic. At present I believe the most likely candidates are the 1572 event seen in Cassiopeia-called "Tycho's Star" because of the observations of Danish astronomer Tycho Brahe, and the great supernova even of 1006-we just passed the 1,000th anniversary of its sighting. That was likely the brightest supernova observed in recorded times, and the distance to its remnant has been measured to be only 7,100 ly.
http://www.noao.edu/outreach/press/pr03/pr0304.html
Edit: I should add that the Cassiopeia A picture is from the seds.org link posted above it, and that it is a Chandra X-Ray Observatory X-Ray image.
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Interesting comment on seds.org
Similar to other galaxies (including the Messier galaxies), there occur supernovae in our Milky Way at irregular intervals of time.
If they are not too heavily obscurred by interstellar matter, they can be seen as very spectacular events in the sky. Unfortunately, though, none of them has been well visible since the invention of the telescope, although modern estimates predict that every few decades one supernova should occur in a galaxy like the Milky Way!
But we haven't seen any.
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Historical Supernovae and their Remnants (http://www.us.oup.com/us/catalog/general/subject/Physics/NuclearParticleAstrophysics/?view=usa&ci=9780198507666) by F. Richard Stephenson and David A. Green.
This book, written by two leading astrophysicists, reviews the historical observations of supernova explosions in our Galaxy over the past two thousand years and discusses modern observations of the remnants of these explosions at radio and other wavelengths.
This seems like an interseting book, although it's a bit pricey at $169.50.
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After a brief hiatus, the Advanced Camera for Surveys aboard NASA’s Hubble Space Telescope is back in business, probing the far reaches of space in a quest to understand the true nature of the universe’s most dominant constituent: dark energy.
One of the first images of the universe was taken after the ACS camera resumed science operation on July 4. The camera was offline for nearly two weeks as NASA engineers switched to a backup power supply after the camera’s primary power supply failed.
An image taken on July 5, shows an apparent supernove when compared to an image taken April 24.
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OK, which came first with the event SN1987a, neutrinos or photons? If the neutrinos arrived first, what are the ramifications for the fact that the speed of light is supposed to be the limit and neutrinos have non-zero rest mass?
http://hyperphysics.phy-astr.gsu.edu/hbase/astro/sn87a.html
Ten neutrino events were detected in a deep mine neutrino detection facility in Japan which coincided with the observation of Supernova 1987A. They were detected within a time interval of about 15 seconds against a background of lower energy neutrino events. A similar facility, IMB in Ohio detected 8 neutrino events in 6 seconds. These observations were made 18 hours before the first optical sighting of the supernova.
Hmmmm.
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I've always wondered about that too. The neutrinos and the photons were in a race that was 160,000 ly long, the neutrinos getting the head start. I guess it must be that the neutrinos, although they had mass and had to travel slower than c, had so much energy as they were blasted out of the star that the difference in speed was negligible. Therefore the neutrinos were able to maintain their lead even over that vast distance.
I think that there are plans in the works to have a space-based supernova observation instrument or instruments to succeed the HST. I don't know what is the most recent status of those plans. Certainly the newer instruments can have a wider field of view, the HST's view is quite narrow and the instrument is suited for observing supernovae already detected rather than for finding new ones. Still I don't know what is on the horizon that can image those very distant objects with the same brightness and resolution as the HST once it is decommissioned.
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I guess that was before they had X-ray/Gamma detectors set up?
I wonder if there was an X-ray/gamma burst that preceded the optical observations.
Also, if the nuclear interactions went on deeper in the star, i.e. below the photosphere/chromosphere, then the neutrinos, with their low interaction probability, could leave the star before the photons, which scatter of ions and electrons, would get out.
I wonder what the effective 'index of refraction' is for a star's interior.
That might explain the neutrinos first - they are moving close to speed of light, but not quite, and they got a head start, by hours or days - maybe.
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I've looked this question over again. I'm not an astrophysicist(!) but I think that what happened is not that the neutrinos got a head start. It's just that in the stellar explosion, there is always a characteristic light intensity curve. That intensity curve reaches a peak over a matter of days, or even weeks. It's not like observing an ordinary explosion, where the initial instant is the brightest. The luminosity of the supernova depends on many things, such as the transparency of the gas envelope around the star's core and, very importantly, the emission area of that envelope. As the gas envelope expands, obviously its area increases quickly.
So I imagine in the first few hours or days following the collapse of the core, which released the neutrinos, the optical brightness was still too low to be noticed. There had to have been a telescope constantly monitoring that progenitor star to have gotten the information that the star had exploded. On the other hand, the neutrino detector is direction insensitive. A big increase in the neutrino background coming from anywhere in the sky would have registered. Neutrino detectors are very inefficient - the fact that eight neutrinos were detected in just six seconds indicates just how gigantic that pulse of neutrinos from that supernova was.
Actually, we need an expert to explain this... :)
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New evidence links stellar remains to oldest recorded supernova
http://www.esa.int/esaSC/SEMGE58LURE_index_0.html
18 September 2006
Recent observations from the European Space Agency's XMM-Newton Observatory and NASA's Chandra X-ray Observatory have uncovered evidence that helps to confirm the identification of the remains of one of the earliest stellar explosions recorded by humans.
The new study shows that the supernova remnant 'RCW 86', observed by XMM-Newton and Chandra, is much younger than previously thought. As such, the formation of the remnant appears to coincide with a supernova observed by Chinese astronomers in 185 AD.
"There have been previous suggestions that RCW 86 is the remains of the supernova from 185 AD," said Jacco Vink of University of Utrecht, The Netherlands, and lead author of the study. "These new X-ray data greatly strengthen the case."
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Towards a Cosmological Hubble Diagram for Type II-P Supernovae
Authors: Peter Nugent (1), Mark Sullivan (2), Richard Ellis (3), Avishay Gal-Yam (3 and 4), Douglas C. Leonard (3 and 5), D. Andrew Howell (2), Pierre Astier (6), Raymond G. Carlberg (2), Alex Conley (2), Sebastien Fabbro (7), Dominique Fouchez (8), James D. Neill (9), Reynald Pain (6), Kathy Perrett (2), Chris J. Pritchet (9), Nicolas Regnault (6) ((1) Lawrence Berkeley National Laboratory, (2) University of Toronto, (3) California Institute of Technology, (4) Hubble Postdoctoral Fellow, (5) NSF Astronomy and Astrophysics Postdoctoral Fellow, (6) LPNHE, CNRS-IN2P3 and University of Paris VI & VII, (7) CENTRA, (8) CPPM, CNRS-IN2P3 and University Aix Marseille II, (9) University of Victoria)
Comments: 36 pages, 16 figures, accepted for publication in ApJ
http://arxiv.org/PS_cache/astro-ph/pdf/0603/0603535.pdf
I am mentioning this because Andy Howell has discovered a distant supernova, or exploded star, so large that it will force scientists to question their understanding of how certain older stars disintegrate. Howell and co-authors Peter Nugent and Richard Ellis will be publishing results and analysis of the discovery in Nature this week.
Supernova leads to questions about death of stars (http://today.reuters.com/news/articlenews.aspx?type=scienceNews&storyID=2006-09-20T205836Z_01_N20216834_RTRUKOC_0_US-SPACE-SUPERNOVA.xml&WTmodLoc=NewsArt-L3-Science+NewsNews-3)
Scientists have believed that dying stars known as "white dwarfs" can't expand to more than 1.4 times the size of our sun without exploding in a massive thermonuclear blast.
That rule, known as the "Chandrasekhar Limit," has served as the foundation of decades of astrophysical research and helped scientists estimate the size of the universe.
But a team of astronomers said on Wednesday that they have found a supernova in a galaxy 4 billion light years away that reached a mass twice that of the sun before exploding.
"It should not be possible to break this limit but nature has found a way," said Andy Howell, the University of Toronto researcher who discovered the supernova.
http://www.news.utoronto.ca/bin6/060920-2575.asp
'Champagne supernova' challenges understanding of how supernovae work
A type Ia supernova breaks Chandrasekhar limit, astronomy's 'standard candles' suddenly variable
An international team of astronomers led by a group at the University of Toronto has discovered a supernova more massive than previously believed possible. This has experts rethinking our basic understanding of how stars explode as supernovae, according to a paper to be published in Nature on September 21.
University of Toronto postdoctoral researcher Andy Howell, lead author of the study, identified a Type Ia supernova named SNLS-03D3bb in a distant galaxy 4 billion light years away that originated from a dense evolved star, termed a 'white dwarf,' whose mass is far larger than any previous example. Type Ia supernovae are thermonuclear explosions that destroy carbon-oxygen white dwarf stars that have accreted matter from a companion star.
Researchers say SNLS-03D3bb’s “obesity” has opened up a Pandora’s box on the current understanding of Type Ia supernovae and how well they can be used for precision cosmology.
Current understanding is that Type Ia supernova explosions occur when the mass of a white dwarf approaches 1.4 solar masses, or the Chandrasekhar limit. This important limit was calculated by Nobel laureate Subrahmanyan Chandrasekhar in 1930, and is founded on well-established physical laws. As such, decades of astrophysical research have been based upon the theory. Yet, somehow the star that went supernova as SNLS-03D3bb reached about two solar masses before exploding.
"It should not be possible to break this limit," says Howell, "but nature has found a way. So now we have to figure out how nature did it."
In a separate News & Views article on the research in the same issue of Nature, University of Oklahoma professor David Branch has dubbed this the “Champagne Supernova,” since extreme explosions that offer new insight into the inner workings of supernovae are an obvious cause for celebration.
The team speculates that there are at least two possible explanations for how this white dwarf got so fat before it exploded. One is that the original star was rotating so fast that centrifugal force kept gravity from crushing it at the usual limit. Another is that the blast was in fact the result of two white dwarfs merging, such that the body was only briefly more massive than the Chandrasekhar limit before exploding. Observations of the supernova were obtained at the Canada-France-Hawaii telescope and the Keck telescope, both located on Mauna Kea in Hawaii.
Since Type Ia supernovae usually have about the same brightness, they can be used to map distances in the universe. In 1998 they were used in the surprising discovery that the universe is accelerating. While the authors are confident that the discovery of a supernova that doesn't follow the rules does not undermine this result, it will make them more cautious about using them in the future.
University of Toronto postdoctoral fellow Mark Sullivan, a coauthor on the research, says, “This supernovae muddies the waters. We now know these rogue supernovae are out there which might throw off our cosmology results if we aren't careful about identifying them.”
SNLS-03D3bb is the first high redshift (z=0.244) Type Ia supernova (SN Ia) with no obvious analogue at low redshift. It is one of the brightest SNe ever observed, with MV=-19.94, and it does not obey the usual lightcurve shape-luminosity relationship for SNe Ia. It also has a SiII velocity at +2 days after maximum light that is among the lowest ever seen -- 8000 km/s . Another unusual feature is evidence in the +2 day spectrum for unburned carbon, which implies turbulent, inefficient burning in the interior of the supernova. This is expected in a deflagration, but unexpected in models with a transition to detonation. PEGASE.2 fits to the host galaxy photometry show it has a high star formation rate ( 1.3 solar masses per year) and a low mass 109 solar masses), indicating a young progenitor. The extreme features of this SN Ia can be explained by an unusually massive progenitor white dwarf.
from Snls-03d3bb: An Overluminous, Low Velocity Type Ia Supernova Discovered At Z=0.244 (http://adsabs.harvard.edu/abs/2006AAS...208.0203H)
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The Supernova Legacy Survey (SNLS)
http://www.cfht.hawaii.edu/Science/CFHLS/MidTermReview/CFHTLS-SNLS-SACreview2005.pdf
http://www.cfht.hawaii.edu/SNLS/
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Type Ia supernovae are really hot items of study right now because it's been presumed they are standard candles that have known actual luminosities regardless of their redshift. The data compiled upon them led the researchers at the Supernova Cosmology Project and the High-Z Supernova group to put forward the theory that the universe's expansion is accelerating. There are other lines of evidence now that support that theory, even if the supernova data has to be re-evaluated.
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New evidence links stellar remains to oldest recorded supernova
http://www.esa.int/esaSC/SEMGE58LURE_index_0.html
I'm not familiar with the details of what is known of the supernova of 185. I took a quick look at Laurence Marschall's "The Supernova Story" where he cited a proposed peak magnitude of -8. That's awfully bright if that was indeed the case, although I don't really know how these "historical astronomers" make such inferences from the scanty data. The data in this case comes from China. But anyway, if it was that bright then it suggests the explosion may not have happened that far away, at least in terms of galactic scale distances.
No information in the link about the distance to this proposed remnant, nor whether it contains a pulsar at the center.
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High Redshift Supernova Search Supernova Cosmology Project (http://panisse.lbl.gov/)
Lawrence Berkeley Labs
Current SCP Supernova search program:
Decelerating and Dustfree: Targeting Supernovae in Very High Redshift Galaxy Clusters with HST (http://panisse.lbl.gov/ACSclustersearch/)
After just over 2 years of operation, the SNLS is already outperforming all previous ground-based high-z SN surveys. The supernova and variable object datasets obtained will have true legacy value. The first SNLS cosmology results are currently inpress (A&A, see poster 15.04) and the SNLS is poised to provide perhaps the most compelling measurement of dark energy achievable over the next decade.
SNLS: Realtime Pipeline and Current Status - lot of good stuff in this poster.
http://snls.in2p3.fr/conf/posters/AAS207/PosterAAS207-Perrett.pdf
and
http://snls.in2p3.fr/conf/posters/AAS207/PosterAAS207-Neil.pdf
Spectra of High-Redshift Type Ia Supernovae and a Comparison with their Low-Redshift Counterparts (http://www.arxiv.org/abs/astro-ph/0509041)
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From the Astronomy Picture of the Day archives, here's a simulated picture of what the great supernova of 1006 looked like shining over the Mediterranean.
http://antwrp.gsfc.nasa.gov/apod/ap030328.html
It was quite possibly the brightest supernova in recorded history, I've seen estimates of its peak magnitude between -7.5 and -9.5. It also appears to have been a Type Ia, in our very own galaxy, practically in our neighborhood. It's distance was calculated to be only 7,100 light years by a research project headed by Frank Winkler of Middlebury College in Vermont. (http://celestialdelights.info/pub/SN1006_1.htm) The rarest of the rare, really, as Type Ia supernovae are usually seen in elliptical galaxies rather than spiral ones like the Milky Way.
The remnant offers little to see nowadays at optical wavelengths, the progenitor star having totally disintegrated and the gas cloud not being bright, but there remains considerable emission from the cloud at long (e.g. radio) and short (e.g. x-ray) frequencies. Here is an X-ray image:
http://wave.xray.mpe.mpg.de/rosat/calendar/1997/jul
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Goddard Supernovae page - http://imagine.gsfc.nasa.gov/docs/science/know_l2/supernovae.html
Imagine shows the night sky before and after SN 1987a. It was big - very big!
http://imagine.gsfc.nasa.gov/Images/icons/adv_snr.gif
There are quite a few extragalactic supernovae recently discovered -
http://www.rochesterastronomy.org/sn2006/novae.html
French Supernovae page -
http://www.astrosurf.com/snaude/index.htm
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Here is some background on SN 185 (which was observed and recorded by Chinese astronomers/philosophers in 185 AD).
Title: Detecting Enriched Ejacta in the Remnant of SN 185 AD
Author: Fesen, Robert A.
http://adsabs.harvard.edu/abs/1987iue..prop.2775F
Title: An X-ray study of the remnant of SN 185 A.D.
Authors: Pisarski, R. L.; Helfand, D. J.; Kahn, S. M.
http://adsabs.harvard.edu/abs/1984ApJ...277..710P
Paper available in pdf format.
Now another thought that came to mind while looking at these 'big explosions' is -
"Do people look at SN emssion/absorption spectra over a period of days/weeks to see what the shift of the emission/absorption lines do?" In other words, to people look at the blue shift in relationship to the red shift. Presumably this gives the velocity of the explosion/chromosphere (or what was the chromoshpere).
Or do people just take a picture and say - Oh look another SN? :-\\
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How to Blow Up a Star (http://www.sciam.com/article.cfm?articleId=000160CC-A71B-150E-A26183414B7F0000)
It is not as easy as you would think. Models of supernovae have failed to reproduce these explosions--until recently
By Wolfgang Hillebrandt, Hans-Thomas Janka and Ewald Müller
On November 11, 1572, Danish astronomer and nobleman Tycho Brahe saw a new star in the constellation Cassiopeia, blazing as bright as Jupiter. In many ways, it was the birth of modern astronomy--a shining disproof of the belief that the heavens were fixed and unchanging. Such "new stars" have not ceased to surprise. Some 400 years later astronomers realized that they briefly outshine billions of ordinary stars and must therefore be spectacular explosions. In 1934 Fritz Zwicky of the California Institute of Technology coined the name "supernovae" for them. Quite apart from being among the most dramatic events known to science, supernovae play a special role in the universe and in the work of astronomers: seeding space with heavy elements, regulating galaxy formation and evolution, even serving as markers of cosmic expansion.
Zwicky and his colleague Walter Baade speculated that the explosive energy comes from gravity. Their idea was that a normal star implodes until its core reaches the density of an atomic nucleus. Like a crystal vase falling onto a concrete floor, the collapsing material releases enough gravitational potential energy to blow the rest of the star apart. An alternative emerged in 1960, when Fred Hoyle of the University of Cambridge and Willy Fowler of Caltech conceived of the explosions as giant nuclear bombs. When a sunlike star exhausts its hydrogen fuel and then its helium, it turns to its carbon and oxygen. Not only can the fusion of these elements release a titanic pulse of energy, it produces radioactive nickel 56, whose gradual decay would account for the months-long after-glow of the initial explosion.
. . . .
http://www.sciam.com/media/inline/000160CC-A71B-150E-A26183414B7F0000_p45.jpg
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That link above describes the modeling of Type Ia supernovae. There are other ideas too, like the possbility that such explosions might occur from the collision of two white dwarf stars.
Type Ia's leave no cinders, such as a neutron star at the center of the explosion such as seen in the Crab Nebula, because the entire core of the star is consumed.
The other supernova types, Ib, Ic, and the various Type IIs, have different models, and their own sets of problems.
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There are other ideas too, like the possbility that such explosions might occur from the collision of two white dwarf stars.
I would be interested in any studies or papers on this possibility. I have to wonder how two white dwarves get together, i.e. how many white dwarves are close enough to collide, and then what is the probability that they would collide - spot on - as opposed to establish a binary system.
I really need to read up on SN's in this galaxy in order to understand the kind of stellar populations involved.
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http://hubblesite.org/newscenter/archive/releases/2006/52/image/e/
Hubble images of 39 distant supernovae.
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Not a supernovae, but supernovae remnants -
A Star's Death Comes to Light (http://www.nasa.gov/centers/marshall/news/news/releases/2007/07-004.html)
Using NASA’s Chandra X-ray Observatory, scientists have created a stunning new image of one of the youngest supernova remnants in the galaxy. This new view of the debris of an exploded star helps astronomers solve a long-standing mystery, with implications for understanding how a star's life can end catastrophically and for gauging the expansion of the universe.
Over 400 years ago, sky watchers -- including the famous astronomer Johannes Kepler -- noticed a bright new object in the night sky. Since the telescope had not yet been invented, only the unaided eye could be used to watch as a new star that was initially brighter than Jupiter dimmed over the following weeks.
Chandra's latest image marks a new phase in understanding the object now known as Kepler's supernova remnant. By combining nearly nine days of Chandra observations, astronomers have generated an X-ray image with unprecedented detail of one of the brightest recorded supernovas in the Milky Way galaxy.
The explosion of the star that created the Kepler remnant blasted the stellar remains into space, heating the gases to millions of degrees and generating highly energized particles. Copious X-ray light, like that shining from many supernova remnants, was produced.
Rather interesting shape.
http://www.nasa.gov/mission_pages/chandra/multimedia/photos07-004.html
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Yes, I saw the news that the NCSU team had compiled evidence that the 1604 supernova was Type Ia. It's really quite remarkable when you think about it - I believe the estimated percentage of Type Ia's among all of the supernovae in the Milky Way galaxy is 20%. Thirty two years earlier there was the famous "Tycho's Star", the supernova of 1572 in Cassiopeia. That has long been thought to have been a Type Ia, a good piece of the evidence comes from the visual observations of Tycho Brahe himself. Two Type Ia's within just about three decades, amazing.
The astronomers of the present day would be deliriously happy to see just ONE Type Ia supernova go off in our galaxy. They are a special class of supernova, and as mentioned in the article, they can serve as standard candles over billions of light years. Yet there is so much that still needs to be learned about them.
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Someone had asked about the production of uranium in supernovae, and I found the following:
http://origins.colorado.edu/uvconf/white_final/node5.html
Stars of all masses spend the majority of their lives fusing hydrogen nuclei into helium nuclei: we call this stage the main sequence. When all of the hydrogen in the central regions of a star is converted into helium, the star will begin to "burn" helium into carbon. However, the helium in the stellar core will eventually run out as well; so in order to survive, a star must be hot enough to fuse progressively heavier elements, as the lighter ones become exhausted one by one. Stars heavier than about 5 times the mass of the Sun can do this with no problem: they burn hydrogen, and then helium, and then carbon, oxygen, silicon, and so on... until they attempt to fuse iron. . . .
The nuclear reactions do not produce enough energy to balance the force of gravity.
. . . . The lack of radiation pressure generated by the iron-fusing core causes the outer layers to fall towards the centre of the star. This implosion happens very, very quickly: it takes about 15 seconds to complete. During the collapse, the nuclei in the outer parts of the star are pushed very close together, so close that elements heavier than iron are formed.
What happens next depends on the mass of the star. Stars with masses between about 5 and 8 times the mass of our Sun form neutron stars during the implosion: the nuclei in the central regions are pushed close enough together to form a very dense neutron core. The outer layers bounce off this core, and a catastrophic explosion ensues: this is the visible part of the supernova.
http://curious.astro.cornell.edu/supernovae.php
http://hyperphysics.phy-astr.gsu.edu/hbase/astro/snovcn.html
http://hyperphysics.phy-astr.gsu.edu/hbase/astro/nucsyn.html#c1
The Mechanism of Core-Collapse Supernovae and the Ejection of Heavy Elements
http://arxiv.org/abs/astro-ph/0212317
http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit3/supernova.html
http://zebu.uoregon.edu/disted/ph123/l10.html
http://universe.nasa.gov/press/2003/030918a.html
http://fias.uni-frankfurt.de/iship2006/talks/thielemann_iship.pdf (presentation)
This one is interesting - I still need to work through it though.
And this one is interesting because of the spectra
http://www.noao.edu/outreach/press/pr00/pr0004.html
http://www.noao.edu/image_gallery/html/im0629.html
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I believe that synthesis of U would have to occur in massive Type II or other core collapse supernovae. The Type Ia's explode by the fusion of C to Fe, there isn't enough energy or starting material to produce the really heavy elements.
One thing I was fascinated to learn is that most of the light from a Type Ia supernova comes from radioactive decay as the radioactive Ni originally ejected from the star decays down to Fe.
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X-Ray Ring Around SN1987A
http://antwrp.gsfc.nasa.gov/apod/ap000512.html
A false-color image from the Chandra X-ray Observatory reveals a one light-year diameter ring of hot, ten million degree plasma. It is one of the most detailed X-ray images of the expanding blast wave from supernova 1987A (SN1987A). At visible wavelengths SN1987A is famous for its evolving rings, and superposed on this image are white contour lines which outline the innermost optical ring as seen by the Hubble Space Telescope.
I'm curious about the asymmetry - why is there asymmetry? Does the image indicate that the left side is closer to us the observers (and the right side is then further)?
:koala
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There may be an answer to the asymmetry question.
Core-collapse, GRBs, Type Ia Supernovae, and Cosmology
http://arxiv.org/abs/astro-ph/0608386
Type Ia Supernovae (SNe) have been used by many to argue for an accelerated expansion of the universe. However, high velocity and polarized features in many nearby SNe Ia, and their inverse relation to luminosity, particularly for polarization, are consistent with an extreme version of the axisymmetry seen in SN 1987A, which could be the result of double degenerate merger-induced core-collapse. This could be the correct paradigm for many SNe Ia and Ic, where Ia's are both thermonuclear and core-collapsed objects, which leave weakly-magnetized, rapidly spinning (~2 ms) pulsars. In this paradigm Ia/c is produced from the merger of two degenerate cores of common envelope WR stars, or of two CO white dwarfs. Its polar blowouts produce the observed high velocity and polarized spectral features in Ia's, and its equatorial bulge is much brighter in Ia's, due to the greater fraction of 56Ni contained within it. These become classified as Ia's when viewed from the merger equator, and Ic's when viewed from the poles, where a hypernova signature and a gamma-ray burst (GRB) will be observed for lines of sight close to the merger axis. Thus cosmology determined strictly from Ia's alone may be flawed because the local sample is selectively biased. Finally, all GRBs start as the short-duration, hard-spectrum variety (sGRBs), after which some are modified to the long-duration, soft-spectrum variety (lGRBs) by interaction with the merger common envelope and previous polar ejection. Thus the initial photon spectrum of all GRBs is known.
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List of Type IIP SN.
http://www.astro.caltech.edu/~avishay/IIP.html
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Interesting paper on Supernovae, which includes a discussion of types.
http://astronomyonline.org/Stars/Papers/AlexNervosaSupernova.pdf
and I'll add this page here
http://astro.uchicago.edu/~vikram/type1a.html
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Star Explosion Highlights "Purple Rose of Virgo" (http://news.nationalgeographic.com/news/2007/03/070329-supernova.html)
March 29, 2007—Lightning may never strike twice in the same place, but star explosions apparently can.
For the second time in 11 years, the galaxy known as NGC 5584 is garnering attention as the brightly colored home of a brilliant supernova.
Located about 75 million light-years away in the direction of the constellation Virgo, NGC 5584 is slightly smaller but similar in shape to our own galaxy, the Milky Way. Scientists suspect that, like the Milky Way, the "purple rose" galaxy houses an energetic and supermassive black hole at its center.
Now that center is being outshined by SN 2007af, the brightest supernova seen so far the year. Visible as a white dot below and to the right of NGC 5584's center, the explosion was detected on March 1 by the European Southern Observatory's Very Large Telescope.
Supernovas of this kind—called Type 1a—usually occur in binary systems, when a compact star called a white dwarf draws matter away from its companion. When it gains enough mass, the white dwarf collapses under the pressure—setting off a cosmic fireworks show and sending matter hurtling off in excess of 9,300 miles (15,000 km) a second.
http://www.supernovae.net/sn2007/sn2007af.html
and they just keep coming - the list is already up to sn2007ba.
There is a page on Extra Galactic Supernovae - http://astrosurf.com/snweb2/
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X-ray evidence supports possible new class of supernova (http://www.esa.int/esaCP/SEM26FSVYVE_index_0.html)
5 January 2007
Evidence for a significant new class of supernova has been found with the European Space Agency's XMM-Newton and NASA's Chandra X-ray Observatory. These results strengthen the case for a population of stars that evolve rapidly and are destroyed by thermonuclear explosions. Such ‘prompt’ supernovas could be valuable tools for probing the early history of the cosmos.
A team of astronomers uncovered a puzzling situation when they examined X-ray data from DEM L238 and DEM L249, the remnants of two supernovas in a nearby galaxy. On the one hand, the unusually high concentration of iron atoms implied that the remnants are the products of thermonuclear explosions of white dwarf stars, a well-known type of supernova known as ‘Type Ia’. On the other hand, the hot gas in the remnants was much denser and brighter in X-rays than typical Type Ia remnants.
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SN 2006gy is in NGC 1260. See more details on -
http://www.rochesterastronomy.org/sn2006/index.html
It had a maximum brightness around 14.9.
NASA claims it as the brightest observed so far -
http://science.nasa.gov/headlines/y2007/07may_bigsupernova.htm?list45222
Now interestingly, there is another bright SN reported this year
SN 2007af was reported in March, and some of the numbers look as bright or brighter than 2006gy.
http://www.rochesterastronomy.org/supernova.html#2007af
http://www.supernovae.net/sn2007/sn2007af.html
http://www.astrosurf.com/snweb2/2007/07af/07afHome.htm
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These topics go together.
Type II Supernovae
http://www.ucolick.org/~bolte/AY4_00/week8/SNeII.html
Evolution of High-Mass Stars
http://www.ucolick.org/~bolte/AY4_00/week8/massive_stars.html
Synthesizing the Chemical Elements
http://www.ucolick.org/~bolte/AY4_00/week8/nucleosynthesis.html
Neutron Stars
http://www.ucolick.org/~bolte/AY4_00/week9/ns-pulsars.html
Type II Supernovae (and the rprocess?)
http://www.phy.anl.gov/theory/zMMW/zFKT/argonne4p.pdf
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I found a nice discussion of Supernovae on Talkorigins.org
http://www.talkorigins.org/faqs/supernova/
It addresses some of the matters mentioned in this thread.
And there are some interesting references with respect to Supernovae, cosmology and neutrinos here:
http://inpa.lbl.gov/INPA/candi.html
http://math.ucr.edu/home/baez/neutrinos.html
http://math.ucr.edu/home/baez/week130.html
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Brightest Supernova May Reignite
New supernova mechanism would set off repeat explosions
http://www.sciam.com/article.cfm?id=brightest-supernova-may-reignite
A new type of ultrapowerful supernova discovered last year may blow its top again, according to a new study.
Researchers report that supernova 2006gy fits a model of star explosion that should have produced two flare-ups already and may culminate in a third before the star fizzles out. A second study proposes that the explosion might have come about from the marriage of multiple stars.
SN 2006gy first caught astronomers' eyes in September. Burning 100 times brighter than a typical supernova, it maintained full strength for an amazing three months, by which point most of its counterparts would have begun fading. Even eight months later it was as brilliant as a so-called type II supernova, the most common variety. The energy unleashed implied that the exploding star was a behemoth of at least 100 solar masses (suns).
To explain SN 2006gy's massive outburst, researchers invoked a competing mechanism called pair-instability, which theoretically kicks in for stars larger than 90 suns. In this scenario, dating to 1967, high-energy gamma rays inside the star convert into pairs of electrons and positrons, draining stellar energy that would normally help maintain its internal pressure, eventually leading to a premature collapse that liberates vast amounts of energy and light. . . . .
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Can you say if there has been supenova recently?
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Can you say if there has been supenova recently?
Yes - this page lists the latest supernovae.
http://www.rochesterastronomy.org/snimages/
It appears in the first week of 2008, there are at least two SN. There are many more last month, and last year (2007).
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This information is a bit dated, but I only dicovered it today.
Recent HRIBF Research - Implications of the HRIBF 17F(p,p)17F Measurement for Nova Nucleosynthesis
http://www.phy.ornl.gov/hribf/news/summer_01.html#4 (http://www.phy.ornl.gov/hribf/news/summer_01.html#4)
Nova explosions (http://www.everything-science.com/sum-01-nova.jpg) are among the largest explosions in the Universe, and are characterized by temperatures over 108 K and densities up to 104 g/cm3. They are responsible for producing much of the 13C, 15N, and 17O found on earth, and are powered by thermonuclear reactions on radioactive nuclei such as 17F(p,gamma)18Ne. The nuclear burning in novae is not dominated by equilibria or steady flow, thus individual reactions can influence both the energy generated and the abundances synthesized in the explosion. For example, the 17F(p,gamma)18Ne reaction rate is thought to influence, among other things, the amount of 15N, 17,18O, and 18F nuclides produced in nova explosions. The 17F(p,gamma)18Ne reaction is also part of a sequence of reactions that provide a possible transition from the hot CNO cycle (http://www.everything-science.com/sum-01-hcno.jpg) to reactions in the rapid proton capture process (rp-process) (http://www.everything-science.com/sum-01-rp.jpg) in the most energetic novae [1].
Refs:
Collaborators include M. S. Smith, D. W. Bardayan, and A. Mezzacappa (ORNL); S. Parete-Koon and M. W. Guidry (U. Tennessee); W. R. Hix (ORNL, U. Tennessee); S. Starrfield (Arizona State).
[1] R. K. Wallace, S.E. Woosley, Astrophys. J. Suppl. 45, 389 (1981)
[2] D. W. Bardayan et al., Phys. Rev. Lett. 83, 45 (1999) (http://pubster.aip.org/journal_cgi/getabs?KEY=PRLTAO&cvips=PRLTAO000083000001000045000001&gifs=yes)
[3] Physical Review Focus, July 9,1999, http://focus.aps.org/v4/st2.html (http://focus.aps.org/v4/st2.html)
[4] D. W. Bardayan et al., Phys. Rev. C 62, 055804 (2000) (http://pubster.aip.org/journal_cgi/getabs?KEY=PRVCAN&cvips=PRVCAN000062000005055804000001&gifs=yes)
[5] R. Sherr and H. T. Fortune, Phys. Rev. C 58, 3292 (1998)
[6] A. Garcia et al., Phys Rev. C 43, 2012 (1991)
[7] M. Wiescher, J. Gorres, and F.-K. Thielemann, Astrophys. J. 326, 384 (1988)
[8] F.-K. Thielemann et al., http://ie.lbl.gov/astro/friedel.html (1995)
[9] W. R. Hix and F.-K. Thielemann, J. Comput. Appl. Math. 109, 321 (1999)
[10] M. Politano et al., Astrophys. J. 448, 807 (1995); S. Starrfield, priv. comm. (2000)
[11] M. S. Smith et al., in "International Conference on Physics with Radioactive Ion Beams - ISOL'01", eds. C. J. Gross, D. J. Dean, M. S. Smith, http://www.phy.ornl.gov/isol01/proceedings/ms/smith.pdf (2001).